Apparatus for obtaining seismic data

Abstract

Apparatus for obtaining seismic data or information in the field comprises electrical communication paths established between seismic detectors distributed over the ground, and a digital seismic data recorder. Each of the paths comprises a pre-amplifier, various filters, a regulable-gain amplifier, and finally a digitizer, or the paths have these elements at least partly in common. The digitizer operates by carrying out random weighing; it comprises a pseudo-random generator and at least part of the contents of this generator is loaded in parallel into a register. The pseudo-random number thus obtained is converted into an analogue signal and is used for weighing a sample of the seismic signal in a comparator. This gives a single bit each time, which is then recorded in the recorder.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. Apparatus for field recording of digital seismic data, comprising: an array of seismic sensors disposed relative to a subsoil structure and capable of delivering respective analog seismic signals in response to a seismic perturbation in said subsoil structure, said sensors being arranged in groups, each group associated with a digitizing unit, and said analog seismic signals containing information about said subsoil structure;   a plurality of digitizing units, each connected to receive the analog seismic signals from a respective said group of sensors;   means for interconnecting said plurality of digitizing units to form a digital communication channel, said digital communication channel having an input and an output;   means coupled for recording digital signals appearing at the output of said digital communication channel;   means for supplying predetermined control signals to the input of said digital communication channel;   controllable means for supplying pseudo-random digital words to the input of said digital communication channel; and   first control means for causing said pseudo-random digital word supplying means to send a plurality of different pseudo-random digital words through said digital communication channel at the outset of each recording operation, each said word being preceded by a said predetermined control signal from said control signal supplying means; and   wherein each said digitizing means comprises:   means including a multiplexer and a variable gain amplifier for processing the analog seismic signals from the associated said group of sensors, and delivering at an output amplified and leveled samples from the respective analog seismic signals in sets, each said set including in sequence a sample of the analog seismic signal from each said sensor of the group;   means for locally generating pseudo-random digital signals by substantially random selection from a predetermined scale of digital reference signals;   means for converting successive said pseudo-random digital signals into corresponding successive analog reference signals which are substantially random between themselves within a predetermined amplitude range;   means for comparing each of said amplified and leveled samples with a respective one of said substantially random analog reference signals and for delivering at an output a one-bit binary signal upon each comparison;   a local digital word register coupled to the output of said comparing means for storing the one-bit binary signals therefrom;   second control means for synchronously operating said multiplexer, said variable gain amplifier, said pseudo-random digital generating means and said digital word register, whereby said digital word register receives sets of said one-bit binary signals corresponding to respective sets of said amplified and leveled analog signal samples and stores said sets as respective seismic digital words;   means having an input and an output, for coupling said digitizing unit to said interconnecting means and for normally transmitting digital signals from said input thereof to said output thereof; and   third control means for (a) initially responding to a said predetermined control signal by causing one of said pseudo-random words applied to the input of the digital communication channel to be received into said local digital word register and loaded in said local pseudo-random generating means, and for modifying the particular control signal responded to so that such control signal will be ignored by the subsequent digitizing units of the digital communication channel; and for (b) subsequently interrupting said normal transmission of digital signals through said coupling means and causing said local digital word register to transfer a said seismic digital word through said output of said coupling means in response to a said predetermined control signal, and for modifying the particular control signal responded to so that such control signal will be ignored by the subsequent digitizing units.   
     
     
       2. Apparatus in accordance with claim 1 in which said control signal supplying means is further capable of supplying an additional control signal to said digital communication channel, and said third control means is adapted to respond to said additional control signal by actuating operation of the associated second control means to produce synchronous operation of said multiplexer, variable gain amplifier, said pseudo-random digital generating means, and said digital word register, whereby said digital word register receives one said set of one-bit binary signals for each said additional control signal applied to said digital communication channel. 
     
     
       3. Apparatus in accordance with claim 1, wherein said variable gain amplifier comprises a stepped gain amplifier having means for gain control sensitive to the level of said seismic signals. 
     
     
       4. Apparatus in accordance with claim 1, wherein said variable gain amplifier comprises an instantaneous floating point amplifier. 
     
     
       5. Apparatus in accordance with claim 1, wherein said variable gain amplifier comprises an amplifier having continuous automatic gain control. 
     
     
       6. Apparatus in accordance with claim 1, wherein said analog signal processing means comprises: means coupled for preamplifying and filtering the analog seismic signals appearing at the output of said seismic sensors and for providing said analog seismic signals in preamplified and filtered form to said multiplexer;   said multiplexer being coupled to receive said preamplified and filtered analog seismic signals and to deliver at an output samples of said analog seismic signals in a repetitive sequence; and   said variable gain amplifier being a sample amplifier coupled to the multiplexer output for amplifying said repetitive sequence of analog signal samples, and including a sample and hold circuit.